ARTICLE
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Selective Hydrogenolysis of Polyols and Cyclic Ethers over Bifunctional Surface Sites on Rhodium Rhenium Catalysts † † ‡ ‡ § § Mei Chia, Yomaira‡ J. Pagan-Torres,‡ David Hibbitts, Qiaohua† Tan, Hien N. Pham, Abhaya K. Datye, Matthew Neurock, Robert J. Davis, and James A. Dumesic*, † Department of Chemical and Biological Engineering, University of Wisconsin-Madison, 1415 Engineering Drive, Madison, Wisconsin 53706, United States ‡ Department of Chemical Engineering, University of Virginia, 102 Engineers’ Way, Post Office Box 400741, Charlottesville, Virginia 22904-4741, United States § Department of Chemical and Nuclear Engineering and Center for Microengineered Materials, University of New Mexico, MSC 01 1120, Albuquerque, New Mexico 87131-0001, United States bS Supporting Information
ABSTRACT: A ReOx-promoted Rh/C catalyst is shown to be selective in the hydrogenolysis of secondary C O bonds for a broad range of cyclic ethers and polyols, these being important classes of compounds in biomass-derived feedstocks. Experi- mentally observed reactivity trends, NH3 temperature-programmed desorption (TPD) profiles, and results from theoretical calculations based on density functional theory (DFT) are consistent with the hypothesis of a bifunctional catalyst that facilitates selective hydro- genolysis of C O bonds by acid-catalyzed ring-opening and dehydration reactions coupled with metal-catalyzed hydrogenation. The presence of surface acid sites on 4 wt % Rh ReO /C x (1:0.5) was confirmed by NH TPD, and the estimated acid site density and standard enthalpy of NH adsorption were 40 μmol g 1 3 3 and 100 kJ mol 1, respectively. Results from DFT calculations suggest that hydroxyl groups on rhenium atoms associated with rhodium are acidic, due to the strong binding of oxygen atoms by rhenium, and these groups are likely responsible for proton donation leading to the formation of carbenium ion transition states. Accordingly, the observed reactivity trends are consistent with the stabilization of resulting carbenium ion structures that form upon ring-opening or dehydration. The presence of hydroxyl groups that reside R to carbon in the C O bond undergoing scission can form oxocarbenium ion intermediates that significantly stabilize the resulting transition states. The mechanistic insights from this work may be extended to provide a general description of a new class of bifunctional heterogeneous catalysts, based on the combination of a highly reducible metal with an oxophilic metal, for the selective C O hydrogenolysis of biomass-derived feedstocks.
1. INTRODUCTION In this contribution, we first present a catalyst pretreatment strategy for obtaining a highly active and stable catalyst consisting A central challenge in biomass conversion lies in developing catalysts for the selective deoxygenation of highly functionalized of rhodium and rhenium supported on carbon (Rh ReOx/C). molecules, such as sugars, polyols, and cyclic ethers, often under This catalyst is then demonstrated to be selective for the 1,2 hydrogenolysis of not only 1 and 3, but more importantly, a conditions where water is used as the solvent. Catalytic deoxy- genation may be accomplished through reactions such as decarbox- wide variety of C O hydrogenolysis reactions previously un- ylation, decarbonylation, dehydration, and C O hydrogenolysis. Of reported in the literature. Mechanistic concepts are then for- these reactions, selective C O hydrogenolysis of polyols and cyclic mulated regarding the nature of the active sites for this catalyst ethers over heterogeneous catalysts represents an important class of system for the hydrogenolysis of cyclic ethers and polyols. In this reactions for the production of high value chemicals, such as the respect, it is shown that the experimental reactivity trends from conversion of 2-(hydroxymethyl)tetrahydropyran 1 to 1,6-hexane- this study can be understood in terms of results from density functional theory (DFT) calculations involving the deprotona- diol 2. Recent experimental work suggests that platinum, ruthenium, rhodium, and iridium catalysts promoted with rhenium display not tion energies (DPEs) of OH groups associated with Rh ReOx only high activity, but also high selectivity in the hydrogenolysis of clusters, combined with the relative stabilities and reaction tetrahydrofurfuryl alcohol 3,3 1,4 and glycerol 45 8 to their corre- energies of carbenium structures formed by acid-catalyzed reac- sponding R,ω-diols. However, despite this growing body of experi- tions and by dehydration and ring-opening activation barriers at mental work, a systematic understanding remains elusive of the fundamental reaction mechanisms governing the rate and selectivity Received: April 26, 2011 of C O bond scission reactions over these interesting catalysts. Published: July 07, 2011
r 2011 American Chemical Society 12675 dx.doi.org/10.1021/ja2038358 | J. Am. Chem. Soc. 2011, 133, 12675–12689 Journal of the American Chemical Society ARTICLE
Table 1. Hydrogenolysis of Tetrahydrofurfuryl Alcohol 3 and 2-(Hydroxymethyl)tetrahydropyran 1 over Oxide-Promoted Rh a Catalysts and Monometallic Catalysts
product selectivity
catalyst: specific rateb catalyst reactant (g:g) time (h) reactant conversion (%) compound (%) (μmol g 1min 1) 4wt%Rh ReOx/C (1:0.5) 1:9 4 3 47.2 1,5-pentanediol 97.2 180 1-pentanol 2.8 1 27.3 1,6-hexanediol 97.0 90 1-hexanol 3.0
4 wt % Rh-MoOx/C (1:0.1) 2:7 4 3 51.6 1,5-pentanediol 91.3 72 1-pentanol 3.2 1-butanol 0.1 othersc 5.4 1 25.8 1,6-hexanediol 88.6 32 1-hexanol 3.5 1-pentanol 7.2 othersc 0.7 4 wt % Rh/C 2:7 4 3 8.5 1,5-pentanediol 59.1 12 1,2-pentanediol 20.7 1-pentanol 7.1 2-pentanol 10.0 othersc 3.1 1 3.3 1,6-hexanediol 43.5 4 1,2-hexanediol 11.1 1-hexanol 2.8 othersc 42.6 d 3.6 wt % ReOx/C 2:7 12 3 NR 1 NRd d 1.8 wt % MoOx/C 2:7 12 3 NR 1 NRd a b fi fi Reaction conditions: 393 K, 34 bar H2. Feed mixtures were 5 wt % reactant in water. Speci c rate de ned as the moles of 1 or 3 reacted per gram of catalyst per minute. c Alkanes in gas phase and monoalcohols at trace levels. d NR: no reaction.
ReOH acid sites on a model 201-atom RhRe cluster. More hydrogenolysis rate of 3 was increased 15-fold over 4 wt % generally, experimental and theoretical results from this work can Rh ReOx/C (1:0.5) compared to the monometallic 4 wt % Rh/ ff be extended to provide a consistent description of a class of C catalyst. The promoting e ect of MoOx is not as pronounced bifunctional heterogeneous catalysts, based on the combination as for ReOx: only a 6-fold increase in the hydrogenolysis rate of 3 of a highly reducible metal with an oxophilic metal, for the was observed for 4 wt % Rh-MoOx/C (1:0.1). Importantly, the selective C O hydrogenolysis of biomass-derived feedstocks. ReOx or MoOx promoters lead to a remarkable enhancement of selectivities to the R,ω-diols for both cyclic ethers: scission of the 2. RESULTS AND DISCUSSION C O bond was observed to occur primarily at the more sterically hindered secondary carbon oxygen bond for the promoted Rh 2.1. Catalyst Formulation and Initial Reactivity Studies. catalysts, resulting in high selectivities to the respective R,ω- A carbon-supported Rh catalyst was modified with two different diols. This behavior is in contrast to nonselective C O hydro- oxophilic promoters, namely ReOx and MoOx, and initial reac- genolysis observed over monometallic 4 wt % Rh/C, as well as tion kinetics studies were carried out using 1 as the reactant metal-catalyzed ring-opening of substituted cyclopentane and (Table S1, Supporting Information [SI]) to identify the amount cyclohexane derivatives over Pt and other transition metals which of promoter required to maximize catalytic activity. For both preferentially occur at the least-substituted carbon carbon promoters used, the specific activity of the promoted Rh catalysts bonds.9 The increases in hydrogenolysis activity and selectivity passes through a maximum as the amount of promoter is for ReOx-promoted Rh/C are in agreement with results reported 4 increased. Catalysts consisting of 4 wt % Rh ReOx/C (1:0.5 by Chen, et al. Experiments using monometallic 3.6 wt % atomic ratio) and 4 wt % Rh-MoOx/C (1:0.1 atomic ratio) were ReOx/C, and 1.8 wt % MoOx/C catalysts indicate that the Re found to be the most active materials, and these catalysts were and Mo promoters in the absence of a highly reducible metal, selected for use in subsequent reaction kinetics studies. such as Rh, do not possess hydrogenolysis activity, and this result ff 3,4,10 The e ect of oxophilic promoters, such as ReOx or MoOx,on is consistent with the literature. the C O hydrogenolysis activities of Rh catalysts is evident in 2.2. Catalyst Characterization. Temperature-programmed experiments with reactants 1 and 3 (Table 1). Specifically, the reduction (TPR) profiles of dried, unreduced catalysts suggest
12676 dx.doi.org/10.1021/ja2038358 |J. Am. Chem. Soc. 2011, 133, 12675–12689 Journal of the American Chemical Society ARTICLE that contact between Rh and the Re and Mo promoters is metallic nanoparticles typically contained both Rh and Re, as established during catalyst preparation (Figure 1), in view of seen in electron dispersive X-ray spectroscopy (EDS) micro- the observation that the H2 consumption peaks for 4 wt % analyses of Figure 2. The average metallic particle size of the 4 wt Rh ReOx/C (1:0.5) and 4 wt % Rh-MoOx/C (1:0.5) coincide %Rh ReOx/C (1:0.5) catalyst before and after reaction was with the H consumption peak of monometallic 4 wt % Rh/C, in estimated to be 2.1 nm from HAADF-STEM analysis of over 2 agreement with results from the literature.11 13 These TPR 700 900 randomly selected particles, as displayed in Figure 3. profiles show that the presence of Rh lowers the reduction The extent of irreversible CO adsorption at 298 K on the temperature of the Re and Mo precursors, suggesting the various supported carbon catalysts was used to estimate the 14,15 fi formation of metal alloys. Quantitative analysis of H2 con- dispersions of the metallic particles (where dispersion is de ned sumption by 4 wt % Rh ReOx/C (1:0.5) reveals that Re is as the fraction of the metallic atoms that are on the surface), and present in a highly reduced state after reduction at temperatures these results are shown in Table 2. The metal dispersion was above 600 K: approximately 0.24 mmol H2 was consumed found to be approximately 50% for both monometallic 4 wt % compared to 0.25 mmol H2 required to completely reduce the Rh/C and 4 wt % Rh ReOx/C (1:0.5). The particle size of the Rh and Re to the metallic state. The low valence of Re in 4 wt % as-prepared 4 wt % Rh ReOx/C (1:0.5) catalyst determined Rh ReOx/C (1:0.5) after reduction with H2 is consistent with from metal dispersion values was found to be in the range of the literature.4,16 2.2 2.3 nm (Table 2). High angle annular dark field scanning transmission electron In general, estimates of the metal particle sizes obtained from microscopy (HAADF-STEM) studies of the 4 wt % Rh ReOx/C HAADF-STEM and metal dispersion values compare well with (1:0.5) catalyst before and after reaction revealed that the one another, and indicate that no significant changes in particle size take place upon exposure to reaction conditions. Temperature-programmed desorption (TPD) profiles for NH3 from reduced catalysts indicate that acid sites are present on 4 wt % Rh ReOx/C (1:0.5) (Figure 4). All catalyst samples fl were pretreated in owing H2 at 523 K for 4 h prior to the adsorption of NH3. As shown in Figure 5, the NH3 desorption fi pro le for 4 wt % Rh ReOx/C (1:0.5) displays a peak with maximum at 498 K. The number of acid sites on 4 wt % μ 1 Rh ReOx/C (1:0.5) was 40 mol g , and number ratio of acid to metal sites was equal to 0.28. Assuming that the standard entropy change of adsorption (ΔS° ) is approximately 150 J ads K 1 mol 1,20,21 which corresponds to loss of the gas-phase translational entropy, and that first-order desorption and rapid Δ ° readsorption occurs, the heat of adsorption of NH3 ( H ads)on 22 4wt%Rh ReOx/C (1:0.5) can be estimated to be 100 kJ 1 mol . In contrast, no discernible NH3 desorption peaks were observed for monometallic 4 wt % Rh/C and 3.6 wt % Re/C Figure 1. Temperature-programmed reduction profiles for (a) 4 wt % under the conditions employed. The lack of acidity of Re/C here Rh/C, (b) 3.6 wt % Re/C, (c) 1.8 wt % Mo/C, (d) 4 wt % Rh Re/C could be due to a combination of low Re loading and poor metal (1:0.5), (e) 4 wt % Rh Mo/C (1:0.5). dispersion.
Figure 2. Representative HAADF STEM images and EDS spot-beam analysis results of (a) as-prepared and (b) spent 4 wt % Rh ReOx/C (1:0.5) catalysts.
12677 dx.doi.org/10.1021/ja2038358 |J. Am. Chem. Soc. 2011, 133, 12675–12689 Journal of the American Chemical Society ARTICLE
Figure 3. Representative HAADF-STEM images and particle size distribution histograms for (a, c) as-prepared and (b, d) spent 4 wt % Rh ReOx/C (1:0.5) catalysts. Number-averaged particle sizes are presented in the figures.
Table 2. Quantification of CO Adsorption on Monometallic and Promoted Rh Catalysts at 298 K
Rh loading Re loading pretreatment irreversible CO uptake dispersion particle size catalyst (wt%) (wt%) Rh:Re (mol:mol) temperature (K) (μmol g 1) CO:Rh (mol:mol) (%)a (nm)b
Rh/C 4 393 151 0.39 52 2.1 Re/C 3.6 723 29 0.15c Rh Re/C (as-prepared) 4 3.6 1:0.5 393 143 0.38 51 2.2 523 145 0.39 51 2.2 723 135 0.36 48 2.3 Rh Re/C (spent) 4 3.6 1:0.5 393 7 0.02 723 74 0.20 a Dispersion calculated with respect to total Rh loading and assuming 0.75 ML coverage of CO at full saturation. b Particle size = 1.1/dispersion; factor of 1.1 calculated from the literature.17 19 c CO:Re.
2.3. Catalyst Pretreatment and stability. Rhenium is ox- 393 K, the reactor was cooled to 298 K, the vessel was ophilic and some of its oxides (e.g., Re2O7 and ReO3) are soluble depressurized, and the reaction solution was filtered to remove in water, leading potentially to the leaching of rhenium from the catalyst. Elemental analysis of the filtrates was performed the catalyst under aqueous reaction environments and to possi- by inductively coupled plasma atomic emission spectroscopy ble homogeneous catalysis. To optimize catalyst pretreatment (ICP-AES). The results in Table S2 (SI) show that the catalytic conditions for maximum activity and stability of the 4 wt % activity and the extent of rhenium leached into the reaction Rh ReOx/C (1:0.5) catalyst, the effect of catalyst reduction solution were highest without pretreatment of the catalyst (2%) temperature on the rate of hydrogenolysis for reactant 1 was or with a low temperature pretreatment (393 K, 1%). The extent first determined, and the extent of rhenium leaching monitored of Re leaching was substantially lower (<0.5%, which is at the (Table S2, SI). These experiments were conducted in a batch detection limit of our system) after pretreatment at 523 K, and reactor, and pretreatment of the dry catalyst was performed therefore, this condition was chosen for use in continuous flow under H2 atmosphere at temperatures of 393 and 523 K prior experiments to further study catalyst stability versus time-on- to addition of the aqueous feed. After reaction for 4 h at stream.
12678 dx.doi.org/10.1021/ja2038358 |J. Am. Chem. Soc. 2011, 133, 12675–12689 Journal of the American Chemical Society ARTICLE
7 > tetrahydrofuran 8. For both classes of cyclic ethers, the absence of the hydroxyl group leads to a significant decrease in C O hydrogenolysis activity. These results suggest that the hydroxyl groups in 1 and 3 are responsible for the high C O hydrogenolysis rates and selectivities to their respective R,ω- diols over 4 wt % Rh ReOx/C (1:0.5). This selectivity toward ring-opening at the more substituted carbon center is not consistent with the preferential ring-opening of the less sterically hindered sites of substituted cyclohexane and cyclopentane intermediates over metal surfaces.9 As is described below in regard to the ring-opening of compounds 5 and 7, metal- catalyzed ring-opening first occurs through dehydrogenation, causing the unsubstituted sites to be preferentially activated. fi We suggest instead that the experimentally observed hydro- Figure 4. NH3 temperature-programmed desorption pro les for (a) genolysis selectivity trends are consistent with a bifunctional 3.6 wt % Re/C, (b) 4 wt % Rh/C, and (c) 4 wt % Rh Re/C (1:0.5). fl 3 1 mechanism in which the catalyst facilitates initial acid-catalyzed Catalyst samples were pretreated in owing H2 (100 cm (STP) min ) at 523 K for 4 h prior to dosing of NH . ring-opening at the more substituted carbon center followed by 3 metal-catalyzed hydrogenation. To elucidate the observed differ- ences in reactivity and product selectivity, a quantitative measure of the reactivity of these different molecules over solid acids is needed. Accordingly, a Born cycle analysis can be used to establish a relationship between the changes in the activation barrier and changes in the catalyst or the molecules reacted. More fi speci cally, the activation barrier (Ea) for a reaction can be written in terms of the deprotonation energy (DPE) of the acid, which provides a measure of solid acidity, the gas-phase carbe- Δ nium ion reaction or formation energies ( Hrxn(R+)), and the Δ interaction energy ( Eint) between the gas-phase carbenium ion and the negative charge of the conjugate base that forms in the transition state:23,24 ¼ þ Δ þ Δ ð Þ Ea DPE HrxnðRþÞ Eint 1
As the catalyst is identical for all of the reactions considered in Figure 5. Results for the hydrogenolysis of 2-(hydroxymethyl) Table 3, the reactions can therefore be described by the changes tetrahydropyran 1 over 4 wt % Rh ReOx/C (1:0.5) in a continu- Δ ous flow system. Conversion of 1 (9), selectivities to 1,6-hexanediol in the gas-phase carbenium ion reaction energies, Hrxn(R+), and Δ 2 (O) and 1-hexanol (2), and reaction rates ( ) at 393 K, 34 bar H , changes in the interaction energies, Eint. If the catalyst is the 2 WHSV = 0.52 h 1, feed: 5 wt % 1 in water. The catalyst was pretreated in same, the changes in the interaction energies that result from fl 3 1 owing H2 (60 cm (STP) min ) at 523 K for 4 h and cooled to the different reactants are often linearly related to the changes in the reaction temperature prior to initiation of liquid feed flow. proton affinity of the reactant or the gas-phase carbenium ion reaction energies.25 The acid-catalyzed C O activation is Figure 5 shows results for the hydrogenolysis of 1 over 4 wt % thought to proceed via the simultaneous protonation of the Rh ReOx/C (1:0.5) in a continuous packed-bed reactor system. oxygen atom in the ring and ring-opening, thus resulting in the fl The catalyst was pretreated under owing H2 for 4 h at 523 K formation of a secondary carbenium which can be stabilized prior to initiating the flow of liquid feed. Under these continuous through a Coulombic interaction between the positive charge on flow conditions, the performance of the catalyst stabilized at the carbenium ion center and the neighboring oxygen on the conversion level of 32% and a selectivity to 2 of 90%. After 120 h hydroxyl group through a back-side interaction as shown in time-on-stream, the activity of the catalyst declined by only 16%. Figure 6. This secondary carbenium ion intermediate can rapidly The stability of the catalyst under continuous liquid flow condi- undergo a hydride transfer with the neighboring CH OH fi d tions provides de nitive evidence that we have established intermediate to form the more stable RCH2 C O(+)H oxo- catalyst pretreatment and reaction conditions under which 4 carbenium intermediate depicted in Figure 6 which more effec- wt % Rh ReOx/C (1:0.5) functions fully as a heterogeneous tively stabilizes the charge on the oxygen atom. Alternatively, the catalyst. ring-opening can proceed directly with a concerted hydride 2.4. Hydrogenolysis of Cyclic Ethers. To better understand transfer from the neighboring CH OH group to form the more d the high hydrogenolysis selectivities of 1 to 2 over 4 wt % stable RCH2 C O(+)H intermediate. While we have calcu- Rh ReOx/C (1:0.5), the hydrogenolysis rates of cyclic ethers lated the reaction energies to form the oxirane (3-membered with different functional groups were studied in a batch reactor, ring) and oxetene (4- membered ring) as well as the RCH2 and the results are shown in Table 3. The reactivity trend for the CdO(+)H oxocarbenium ions that result from the stabilization six-membered cyclic ethers was observed to be 1 . 2-methylte- of the OH intermediates (see Table 4), we herein discuss trahydropyran 5 > tetrahydropyran 6. The reactivity trend of the only the results for the energetically more stable oxocarbenium five-membered ring cyclic ethers is analogous to that of the six- ion intermediates. To further demonstrate and ensure the membered ring molecules, where 3 . 2-methyltetrahydrofuran validity of gas-phase carbenium ion calculation results, more
12679 dx.doi.org/10.1021/ja2038358 |J. Am. Chem. Soc. 2011, 133, 12675–12689 Journal of the American Chemical Society ARTICLE
a Table 3. Hydrogenolysis of Cyclic Ethers and Polyols over 4 wt % Rh ReOx/C (1:0.5)
a b fi fi Reaction conditions: 393 K, 34 bar H2, mass ratio of catalyst:reactant = 1:9. Speci c rate de ned as the moles of reactant consumed per gram of catalyst per minute. Total specific rates are divided by two to account for symmetry in molecules 11, 12, 15, 16, and ethylene glycol. c NR: no reaction. d Mass ratio of catalyst:reactant = 2:7. e Alkanes in gas phase and monoalcohols at trace levels. f Ring-closing rate denoted in parentheses. g Catalyst used was 4 wt % Rh/C. detailed analyses of the reaction paths and corresponding The results from DFT calculations for simple gas-phase activation barriers were performed for the five-membered carbenium ion reaction energies presented in Figure 6 and Table 4 ring structures 3 and 7 over a model Rh Re OH cluster, as show that the high selectivities for ring-opening 1 and 3 to the 200 1 outlined below. R,ω-diol can be attributed to the greater stability (30 kJ mol 1)of
12680 dx.doi.org/10.1021/ja2038358 |J. Am. Chem. Soc. 2011, 133, 12675–12689 Journal of the American Chemical Society ARTICLE
Figure 6. DFT-calculated carbenium and oxocarbenium ion formation or reaction energies for 2,3-butanediol 16, 2,4-pentanediol 15, and tetrahydrofurfuryl alcohol 3. The dotted lines refer to sequential paths which proceed via the formation of the OH-stabilized three (oxirane) and four (oxetene) ring intermediates that subsequently form the corresponding oxocarbenium ion intermediates, whereas the solid lines refer to concerted protonation/hydride transfer steps that result in the direct formation of the oxycarbenium ion intermediates. the secondary carbenium ion structures leading to the R,ω-diol, The ring-opening of cyclic ethers 5 and 7 is characteristically versus the primary carbenium ion structures which result in the different from the functionalized cyclic structures discussed formation of the 1,2-diol. Furthermore, the presence of a hydroxyl above, in that the C O activation occurs at the sterically less- group at the R or β position allows for the concerted protonation hindered primary carbon oxygen bond, leading to the forma- and hydride transfer to form the more stable oxocarbenium tion of the corresponding secondary alcohols (i.e., 2-hexanol 9 (RCH2 CH=O(+)H) ion intermediate. For cyclic ethers, the and 2-pentanol 10, respectively). This preferential hydrogeno- presence of the hydroxyl substituent R to the dehydration center lysis of 7 at the primary carbon oxygen bond is similar to that is shown to increase the stability of the carbenium ion, such that observed by Gennari, et al. who performed gas-phase reactions the carbenium ion formation energies for 1 and 3 are 845 and over unpromoted Pt catalysts.26,27 The activation of the ring for 852 kJ mol 1, respectively, while the corresponding structures these cyclic ethers, however, cannot be explained through acid without the R OH substituent, 5 and 7, are significantly less chemistry, as it would result in the formation of a highly unstable stable at 742 and 743 kJ mol 1, respectively (Figure 6 and primary carbenium ion. Instead, these experimental trends are Table 4). This trend was confirmed by more rigorous transition consistent with DFT results for ring-opening of cyclic ethers via a state simulations over model Rh200Re1OH clusters, displayed in solely metal-catalyzed route over Rh (Figure 9), showing that Figure 7, which show that the activation barrier to ring-open 3 metal-catalyzed insertion is favored at the less substituted C O (Figure 7A) is lower by 14 kJ mol 1 compared to the barrier to bond by 30 kJ mol 1 due to steric hindrance at the substituted open 7 (Figure 7C) as a result of the back-side stabilization of the C O bond, preventing the initial dehydrogenation which leads positive charge in the carbenium ion transition state by the OH to C O cleavage. group and by stabilization with water. The concerted protonation 2.5. Hydrogenolysis of Polyols. To study further the role of and hydride transfer reaction to form the oxocarbenium ion hydroxyl groups in the hydrogenolysis of neighboring C O transition state lowers the activation barrier by an additional bonds, the reactivity profiles for several straight-chain polyols 1 16 84 kJ mol as shown in the Results displayed in Figure 8. over the 4 wt % Rh ReOx/C catalyst were examined, and these While these calculations explain the selectivities reported in results are shown in Table 3. For all diols studied, it is apparent Table 3, the absolute values of the activation barriers are likely that 1,2-diols are more reactive than R,ω-diols. Furthermore, the overpredicted due to the simplicity of the model system; both the selectivity is significantly higher toward hydrogenolysis of the presence of water and acid sites stronger than those in the hydroxyl group at the secondary carbon atom for all 1,2-diols, modeled system would lead to lower activation barriers. leading to the formation of the corresponding primary alcohol as
12681 dx.doi.org/10.1021/ja2038358 |J. Am. Chem. Soc. 2011, 133, 12675–12689 Journal of the American Chemical Society ARTICLE
Table 4. DFT Calculated Gas-Phase Reaction Energies for the Formation of Carbenium and Oxocarbenium Ion Intermediates Involved in the Ring-Opening of Cyclic Ethers and Dehydration of Polyols
the predominant product. The high reactivity for hydrogenolysis reactants. The higher reactivity and selective hydrogenolysis of of a secondary hydroxyl group adjacent to a primary hydroxyl 1,2-diols, such as 1,2-pentanediol 13 and 1,2-butanediol 14,at group is also exhibited in the reactivity trends of the triols. In the secondary C O bond can be explained by the formation of addition, a secondary hydroxyl group in close proximity to the more stable RCH2 CH=O(+)H oxocarbenium ion inter- another secondary hydroxyl group shows high reactivity for mediates resulting from the concerted elimination of water and undergoing hydrogenolysis over 4 wt % Rh ReOx/C (1:0.5). hydride transfer from the primary CH2OH group, as reported These reactivity trends show that the position of hydroxyl groups in Table 4. Additionally, the higher reactivities of β-diols such as in a reactant molecule is instrumental in dictating its hydro- 2,4-pentanediol 15 and 2,3-butanediol 16 over that for 1,2-diols genolysis reactivity over 4 wt % Rh ReOx/C (1:0.5). can be similarly explained by the formation of the secondary As previously shown for cyclic ethers, the above experimental oxocarbenium ions in 15b and 16b which are more than 100 kJ trends for polyols are also consistent with a bifunctional mol 1 more stable than the secondary carbenium ions in 15a catalyst which facilitates initial acid-catalyzed carbenium ion and 16a. chemistry followed by metal-catalyzed hydrogenation. The low 2.6. Governing Principles of Substrate Reactivity and hydrogenolysis rates of R,ω-diols 2, 1,5-pentanediol 11, and 1,4- Selectivity. The experimental reactivity and selectivity trends butanediol 12 are in agreement with the lower stability of primary shown in Table 3 are in agreement with the carbenium ion carbenium ions which would form upon dehydration of these formation energies presented in Table 4. The reactants studied
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Figure 7. DFT-calculated reactant and transition state structures and corresponding activation barriers for the acid catalyzed ring-opening of: (a) water- stabilized tetrahydrofurfuryl alcohol 3, (b) tetrahydrofurfuryl alcohol 3 and (c) 2-methyltetrahydrofuran 7 on a model Rh200Re1OH cluster.
Figure 8. DFT-calculated reactant and transition state structures for the concerted protonation, hydride transfer and ring-opening of the water- stabilized tetrahydrofurfuryl alcohol 3 over a model Rh(111) surface with well dispersed Re OH sites. are ranked from the most to the least reactive in Table S3 (SI), most reactive structures examined with specific reaction rates of along with their corresponding carbenium ion reaction energies. 90 and 180 μmol g 1 min 1, respectively. The results indicate that the most reactive molecules are the β-Diols (15 and 16)alsoleadtostableoxocarbeniumion cyclic ethers which contain hydroxyl substituents that are located structures that can effectively stabilize the carbenium ion charge R to the carbon atom in the C O bond where the ring is opened. that results upon dehydration. These structures appear to be just as These reactants can undergo concerted ring-opening and hy- reactive as those for 1 and 3 with specific rates of 58 (15)and78 μ 1 1 dride transfer from the neighboring CH2OH group to form (16) mol g min . In agreement with this experimental stable primary oxocarbenium ion transition state structures finding, the theoretical results show similar carbenium ion forma- which are further stabilized by the OH groups that result upon tion energies. The structures of carbenium ions from linear polyols protonation of the ring. As such, structures 1 and 3 are two of the with R,β OH groups are less stable, as the oxocarbenium ions
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that form are now delocalized over primary carbon atoms in the expressed as the ratio between specific reaction rate (μmol g 1 oxocarbenium ion structure without the additional OH stabiliza- min 1) and initial reactant concentration (μmol mL 1). This tion in the transition state; the reactivities of these reactants ratio is employed to reflect an appropriately normalized reaction lie between 23 and 62 μmol g 1 min 1. Linear polyols that do rate which is independent of the minor differences in the initial not contain R,β or β,β OH substituents are unable to form molar concentrations of reactants (see below). It is also notable oxocarbenium ion structures and must effectively stabilize the that specific rate values were divided by two to account for charge on the secondary carbon centers. As such the reactivities molecules with a plane of symmetry, namely, 11, 12, 15, 16, and of these reactants are significantly lower and found to be in the ethylene glycol. The differences in reactivity between unsubsti- range of 10 μmol g 1 min 1. Finally, all of the primary carbenium tuted (primary carbenium ions), substituted (secondary), and ion centers from linear and cyclic polyols are unstable, and as such substituted and OH-stabilized cyclic ethers and polyols are these reactants do not show significant reactivity. highlighted by different colored regions in Figure 10. The results from Table S3 (SI) (with the exception of the data We note here that we have included glycerol in the correlation for the ring-opening of 5 and 7 which are metal-catalyzed of Figure 10, showing that the overall reactivity for conversion of reactions, and data from the unreactive R,ω-polyols) are plotted this molecule is controlled by acid catalysis, consistent with the in Figure 10, showing a correlation between the carbenium ion high selectivity for the formation of 1,3-propanediol 17, com- formation energy and the logarithm of the normalized rate pared with the metal-catalyzed route over Rh/C. It is interesting that the rate of formation of 1,2-propandiol over the Rh ReOx/ C catalyst is also much higher than the corresponding reaction over Rh/C, suggesting that an acid-catalyzed pathway exists involving a secondary carbenium ion along the way to form the 1,2 oxirane that can subsequently react to form both 1,2 and 1,3 propanediol products close to the relative percentages that are seen experimentally. 2.7. Reaction Kinetics. Although it was impractical to evaluate the complete reaction rate expression for all 15 of the different substrates in Table 4, we evaluated the reaction kinetics for our prototypical cyclic ether, 2-(hydroxymethyl)tetrahydropyran (compound 1), and made the reasonable assumption that other substrates in this study will behave similarly. Importantly, we used a continuous flow reactor for this study to eliminate any possible complications that could be caused by initial transient phenomena (e.g., catalyst deactivation, initial leaching of rhenium) that could take place in a batch reactor. The results of these Figure 9. DFT-calculated reaction path and energies for the metal- flow reactor studies, presented in Table S4 (SI), show that the catalyzed ring-opening of 2-methyltetrahydrofuran 7 at the substituted reaction is approximately first order with respect to species 1 and and unsubstituted carbon centers over Rh. slightly less than first order (i.e., approximately equal to 0.9) with
Figure 10. Comparison of the normalized rate, defined as the ratio between specific reaction rate (μmol g 1 min 1) and initial reactant concentration (μmol mL 1) (on a logarithmic scale), and DFT-calculated carbenium and oxocarbenium ion energies for selected cyclic ethers and linear polyols. Each region shows distinct shifts in activity due to the stability of the carbenium and oxocarbenium ions as discussed in Table S3 (SI).
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Figure 11. DFT-calculated deprotonation energies for various surfaces and atom clusters. respect to hydrogen pressure. Because the reactivity trends of this given the fact that rates vary by more than an order of magnitude paper indicate that acid sites must be involved in a kinetically across the series of compounds. significant part of the overall reaction scheme, one might have 2.7. Role of Rhenium and Nature of the Active Site. We expected the reaction to be zero order in hydrogen. However, the suggest that the importance of acid chemistry over the rho- observed positive order is consistent with the bifunctional nature dium rhenium catalyst originates from the Brønsted acidity of the catalyst and the complexity of the mechanism, as illustrated of hydroxylated Re atoms on Rh Re nanoparticles, arising from by Boudart and Djega-Mariadassou in studies of the influence of strong Re O bonds, resulting in a weak O Hbondaswell hydrogen on the kinetics for isomerization of n-pentane to as high electron affinity for the conjugate base. Other solid isopentane over a bifunctional Pt/alumina catalyst (containing acids such as zeolites23 and heteropolyacids (HPAs)29 achieve this metal sites and acid sites), even though hydrogen does not appear high electron affinity through the distribution of the negative in the overall stoichiometric equation of alkane isomerization.28 charge across multiple oxygen atoms. As shown in Figure 11, In that example, the rate determining step is the acid-catalyzed hydroxylated rhenium species on Rh Re nanoparticles demon- isomerization of n-pentene to i-pentene, and the observed strate deprotonation energies at the corner, edge and terrace sites effect of the hydrogen pressure is caused by the equilibrium of of 1140 kJ mol 1, as compared to values of 1050 1200 kJ mol 1 n-pentane with n-pentene on the metal sites prior to the rate for well-known solid acids such as zeolites23 and HPAs.29 determining acid-catalyzed isomerization step. In our case, we A more detailed analysis of the electronic structure shows that observed positive order in hydrogen resulting from the required there is electron transfer from Re to the oxygen. Also, it is evident hydrogenation of an unsaturated intermediate after the acid- from Figure 11 that the DFT-predicted acidities of hydroxylated catalyzed ring-opening step. In addition, it is possible that a molybdenum species on Rh Mo nanoparticles are markedly positive effect of the hydrogen pressure is also caused by the lower than that for Rh Re, consistent with the lower hydro- d hydrogenation of Re O species to form the acidic Re OH genolysis rates observed of 1 over the 4 wt % Rh-MoOx (1:0.1) groups that are required for the overall reaction. catalyst (Table 1). Further experimental evidence of the presence In summary, a simple first order reaction rate constant cannot of surface acid sites on the Rh Re catalyst is provided by be calculated for the reactions in this paper since they occur over results from NH TPD studies (Figure 4) which show that the 3 a bifunctional catalyst containing acid sites and metal sites, both acid site density over this catalyst is approximately 40 μmol g 1. of which contribute to the overall reaction. However, because the Additionally, we found that the hydrogenolysis of 1 over fi reaction appears to be approximately rst order with respect to monometallic Rh/C in the presence of 0.1 M H2SO4 or HCl the concentration of the organic reactant, we have instead chosen proceeds at rates and selectivities to 2 which are significantly to correlate observed rates divided by the reactant concentration, lower than that over RhReOx/C (Table 5). The above results to approximate the rate at nearly identical experimental condi- thus suggest that the active sites for RhReOx/C are located at the tions. The trend in Figure 10 of this normalized rate versus the catalyst surface in the form of hydroxylated rhenium near calculated carbenium ion formation energies is surprisingly good, rhodium.
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Table 5. Effect of Homogeneous Acid and Base on Hydrogenolysis Rate and Product Selectivity of 2-(Hydroxymethyl)- a tetrahydropyran 1 over Rh-Containing Catalysts
selectivity to selectivity to specific rateb catalyst time (h) conversion (%) 1,6-hexanediol 2 (%) 1,2-hexanediol (%) (μmol g 1 min 1) 4wt%Rh ReOx/C (1:0.5) 4 27 97 0 90 c 4wt%Rh ReOx/C (1:0.5) and 0.1 M NaOH 4 NR 4 wt % Rh/C 20 NRc
4 wt % Rh/C and 0.1 M H2SO4 20 13 41 1 9 4 wt % Rh/C and 0.1 M HCl 20 6 32 7 4 4 wt % Rh/C and 0.1 M NaOH 20 NRc a b fi fi Reaction conditions: 393 K, 34 bar H2, mass ratio of catalyst: 1 = 1:9. Reactant mixtures were 5 wt % 1 in water. Speci c rate de ned as the moles of 1 reacted per gram of catalyst per minute. c NR: no reaction
We propose that the bifunctional nature of the RhReOx/C activity of Rh ReOx/C decreases when the catalyst is pretreated fl catalyst described above can be generalized to other transition in owing H2 at increasing temperatures. These results again metal catalysts containing rhenium. For example, we have indicate that the active sites contain oxygen species associated recently reported the promotion of the hydrogenolysis of 4 on with the oxophilic Re promoter of the Rh ReOx/C catalyst. We 8 Re-promoted Pt catalysts. In that work, addition of Re to the Pt note here that we have chosen a pretreatment temperature in H2 catalyst increased the turnover frequency of the hydrogenolysis of 523 K for the Rh ReOx/C catalyst in our studies (see of 4 by a factor of 20, with 17 now appearing in the product Figure 5), as a compromise between retaining oxygen species mixture. Indeed, the selectivity of the hydrogenolysis reaction to associated with Re and also preventing leaching of highly 17 over Pt Re was 34% at 443 K, a temperature at which Pt oxidized Re species into the aqueous solution during reaction. alone exhibited only trace conversion of glycerol. Similarly, we observe here that the hydrogenolysis of 4 over 4 wt 3. CONCLUSIONS %Rh Re/C (1:0.5) resulted in a marked increase in hydrogeno- lysis rates and selectivities to 17 compared to the performance of We have demonstrated in this work that a ReOx-promoted monometallic 4 wt % Rh catalyst (Table 3). The higher selectiv- Rh/C catalyst is selective for the hydrogenolysis of secondary ities to 17 in the presence of rhenium are consistent with the C O bonds for a broad range cyclic ethers and polyols. It is also premise that monometallic Pt or Rh are only able to facilitate shown that a catalyst pretreatment strategy of reducing the metal-catalyzed C O scission, while the addition of rhenium catalyst in H2 at 523 K prior to contact with the aqueous reactant results in a bifunctional catalyst which is also capable of acid results in the attainment of high catalytic activity and stability in a chemistry. King, et al.30 suggested the possible formation of acid continuous flow reaction system. Importantly, results from sites associated with the presence of ReOx clusters in PtRe/C experimentally observed reactivity trends, NH3 TPD, and DFT catalysts, and they suggested the possible influence of these species calculations are consistent in supporting the hypothesis of a in the aqueous phase reforming of glycerol. The basis of this bifunctional catalyst which facilitates acid catalyzed ring-opening proposed acidity over Pt Re catalysts is clearly demonstrated by and dehydration coupled with metal-catalyzed hydrogenation. the DFT data presented in Figure 11, wherein Pt Re nanopar- The presence of surface acid sites on 4 wt % Rh ReOx/C (1:0.5) fi ticles are shown to display acidities comparable to that for Rh Re has been con rmed by NH3 TPD, and indicates that the acid site nanoparticles, suggesting that similar acid-catalyzed chemistry density and standard enthalpy of NH3 adsorption are approxi- 1 1 would be displayed in both systems. mately 40 μmol g and 100 kJ mol , respectively. Results Our proposed reaction scheme involving acid-catalyzed from DFT calculations show that hydroxyl groups on rhenium dehydration at the secondary carbon of a polyol, followed by hydro- associated with rhodium are acidic, thus enabling proton dona- genation to form the R,ω-diol, is consistent with the exper- tion to reactant molecules and formation of carbenium ion imental result that the presence of NaOH completely suppressed transition states. The observed reactivity trends and DFT the hydrogenolysis activity of 1 over 4 wt % Rh ReOx/C (1:0.5) calculations are also consistent with the enhancement in stability (Table 5). Also, previous work involving the hydrogenolysis of of carbenium and oxocarbenium ion structures that form as a 4 over PtRe showed that addition of NaOH led to suppression of result of protonation and concerted protonation and hydride 20 formation.8 Other investigators have also shown that promo- transfer steps, respectively. The experimentally obtained rates of tion of Rh31 or Ru32 with Re enhances the formation of 17 from hydrogenolysis can be correlated with DFT-calculated carbe- 4 in neutral water. Moreover, examination of the LIII edge of Re nium ion formation energies. Results from this study provide the in PtRe8 as well as RhRe31 bimetallic catalysts for the hydro- first consistent account for the bifunctional nature of active sites genolysis of 4 confirmed that Re is not reduced entirely to the in metal catalysts promoted with oxophilic additives for hydro- metallic state after mild reduction in H2, but instead remains in a genolysis reactions, providing guidance for the use of this class of partially oxidized state. heterogeneous catalysts for the selective deoxygenation of bio- Finally, we note that it is possible to destroy the active sites for mass to fuels and chemicals. Rh ReOx/C catalysts by reduction in H2 at elevated tempera- tures. In particular, the results in Figure 1 from temperature 4. METHODS AND MATERIALS programmed reduction studies indicate that the Rh and Re in the Rh ReOx/C catalyst are both highly reduced at temperatures Catalyst Preparation. Catalysts were prepared by incipient wet- above about 600 K. In addition, we have found that the catalytic ness impregnation of Vulcan XC-72 with aqueous solutions of RhCl3,
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NH4ReO4, and (NH4)6Mo7O24 3 4H2O. Rh/C catalysts with oxophilic (i.e., 393 K), the aqueous reactant mixture pumped into the vessel using additives were obtained by successive impregnation of dried, unreduced a HPLC pump (LabAlliance, Series 1), and mechanical stirring of the Rh/C with the corresponding precursor of the oxophilic additive. The reaction mixture initiated. loading amount of the second metal is stated as a atomic ratio (e.g., 4 wt Continuous Flow Reaction Experiments. The flow reactor was %Rh ReOx/C (1:0.5) catalyst designates a metal atomic ratio of Rh: a 6.4 mm (0.25 in.) outer diameter stainless steel tube with wall thickness Re = 1:0.5). Prior to use in experiments, catalysts were dried in air (393 K), of 0.7 mm (0.028 in.). The catalyst bed consisted of reduced and reduced in flowing H2 (723 K), and passivated with flowing 2% O2 in He passivated catalyst (0.35 g) loaded between a quartz wool plug and fused “ ” (298 K). Reduced and passivated catalysts are described as as-prepared SiO2 granules ( 4 + 16 mesh, Sigma-Aldrich). The reactor was heated catalysts in this paper. with a furnace composed of close-fitting aluminum blocks externally CO Adsorption. Prior to CO adsorption measurements, catalysts heated by an insulated furnace (1450 W/115 V, Applied Test Systems 3 1 were pretreated in 150 cm (STP) min flowing H2 at 393, 523, or 723 series 3210), and temperature was monitored using a K type thermo- K for 4 h. The CO adsorption uptakes at 298 K were measured on a couple (Omega) placed outside the reactor immediately next to the standard gas adsorption apparatus described elsewhere.33 position of catalyst bed. Temperature was controlled with a PID Temperature-Programmed Reduction and NH3 Desorp- temperature controller (Love Controls, series 16A). Liquid feed solution tion. Temperature programmed experiments were carried out using was introduced into the reactor with cofeeding of H2 in an up-flow an apparatus comprising of a tube furnace connected to a variable power- configuration. A HPLC pump (LabAlliance, series 1) was used to supply and PID temperature controller (Love Controls) with a K-type control the liquid feed rate. Gas flow was controlled with a mass-flow thermocouple (Omega). For temperature-programmed reductions, the controller (Brooks 5850 model), and the system pressure was main- reducing gas consisted of 5% H2 in He, at a flow-rate of approximately tained at 34 bar by passing the effluent gas stream through a back- 15 cm3(STP) min 1. Dried, unreduced catalyst samples (0.2 g) were pressure regulator (GO Regulator, model BP-60). The effluent liquid loaded into a 12.6 mm outer diameter, fritted quartz tube, purged with was collected in a gas liquid separator and drained periodically for the reducing gas for 30 min, followed by initiation of a temperature ramp analysis by HPLC. Carbon balances closed within 5% for all data points. 1 at 10 K min to 973 K. For NH3 temperature-programmed desorption Analytical Methods. Quantitative analyses were performed using studies, reduced and passivated catalyst samples (0.4 g) were loaded into a Waters 2695 separations module high performance liquid chromatog- 3 1 the fritted quartz tube, pretreated in flowing H2 (100 cm (STP) min ) raphy instrument equipped with a differential refractometer (Waters at 523 K for 4 h, and purged with He (200 cm3(STP) min 1) at 523 K 410), a photodiode array detector (Waters 996), and an ion-exclusion for 90 min. NH3 adsorption was performed at 298 K using 1% NH3 in column (Aminex HPX-87H, 300 mm 7.8 mm). 0.005 M H2SO4 at a 3 1 1 He at a flow-rate of 100 cm (STP) min . After NH3 adsorption, the flow-rate of 0.6 mL min was used as the mobile phase. Gas-phase sample was purged with flowing He (200 cm3(STP) min 1) at 298 K for products from batch reactions were collected in a gas bag and analyzed 90 min. The He flow-rate adjusted to approximately 30 cm3(STP) using a Varian GC (Saturn 3) equipped with a FID detector and a GS-Q min 1 followed by initiation of a temperature ramp at 10 K min 1 from column (J&W Scientific). The presence of rhenium in solution was 298 to 1073 K. The effluent from the reactor tube was introduced into a detected using inductively coupled plasma atomic emission spectro- vacuum chamber (5 10 5 Torr) via a constricted quartz capillary, and metry (ICP-AES). the composition of the gas was monitored by a mass spectrometer Density Functional Theory Calculations. All gas-phase ener- system (quadruple residual gas analyzer, Stanford Instruments RGA gies were calculated using gradient-corrected DFT calculations as 3,14,34,35 200). Vacuum for the mass spectrometer was provided by a diffusion implemented in DMol. Wave functions were represented by pump connected in series to a rotary pump. The NH3 desorption numerical basis sets of double numerical quality (DNP) with d-type profiles reported are based on the mass 15 (NH) signal which was polarization functions on each atom and expanded out to a 3.5 Å cutoff. 36 verified to be directly indicative of NH3 desorption alone and did not The Perdew Wang 91 (PW91) form of generalized gradient approx- experience any interference from the fragmentation of contaminants imation (GGA) was used to model gradient corrections to the correla- such as H2O and CO2. tion and exchange energies. The electronic density for each self- 5 Electron Microscopy. Catalysts were dispersed in ethanol and consistent iteration converged to within 1 10 au. The energy in 5 mounted on holey carbon grids for examination in a JEOL 2010F 200 kV each geometry optimization cycle was converged to within 2 10 3 transmission electron microscope equipped with Oxford Energy Dis- Hartree with a maximum displacement and force of 4 10 Å and 3 persive Spectroscopy (EDS) system for elemental analysis. Images were 3 10 Hartree/Å, respectively. recorded in high angle annular dark field (HAADF) mode with a probe The gas-phase carbenium ion reaction or formation energies were size of 1.0 nm. calculated for ring structures as: Spent Catalyst Samples. All spent catalyst samples were obtained þ þ ROR þ H f RORH from batch reactor measurements for the hydrogenolysis of 1 (393 K, 34 + bar H2, 4 h, mass ratio of catalyst:1 = 1:9). Reaction mixtures were where ROR and RORH refer to the initial ring structure and the ring- filtered immediately after cooling and depressurization of the reactor. opened carbenium ion. The energies for the linear polyols, however, Catalyst samples were vacuum-dried at room temperature prior to use. result in dehydration and, as such, were calculated as: Batch Reactor Studies. Reactions were carried out using a 50 mL þ þ f þ þ pressure vessel (Hastelloy C-276, model 4792, Parr Instrument). After ROH H RH H2O loading the reactant solution, catalyst, and magnetic stirrer bar into the + reactor, the vessel was sealed, purged with 34 bar He, and pressurized where ROH and RH refer to the initial polyol and the resulting dehydrated carbenium ion. Molecules which contained neighboring R with H2 (34 bar). Mechanical stirring was maintained using a magnetic β stirrer plate (500 rpm). or OH substituents were found to preferentially undergo a concerted protonation and hydride transfer to stabilize the resulting product: Catalyst Pretreatment in Batch Reaction Studies. Pretreat- ment of the dry catalyst with H2 was carried out at temperatures of 393 R CH2OH CH2OH f ½R CHðþÞ CH2OH and 523 K for 12 h. Reduced and passivated dry catalyst was pretreated in f R CH CHdOðþÞH a 50 mL pressure vessel (Hastelloy C-276, Model 4792, Parr In- 2 strument) under H2 atmosphere (34 bar) at the stipulated pretreatment The gas-phase carbenium ion energies are used herein only as an temperature. The reactor was then cooled to the reaction temperature initial probe of the activity and selectivity of the different molecules
12687 dx.doi.org/10.1021/ja2038358 |J. Am. Chem. Soc. 2011, 133, 12675–12689 Journal of the American Chemical Society ARTICLE examined experimentally. This approach oversimplifies the environment (4) Chen, K.; Koso, S.; Kubota, T.; Nakagawa, Y.; Tomishige, K. of the metal in solution. While the presence of the aqueous solution will ChemCatChem 2010, 2 (5), 547–555. clearly lower the overall reaction energies and activation barriers, we do (5) Nakagawa, Y.; Shinmi, Y.; Koso, S.; Tomishige, K. J. Catal. 2010, not believe that selectivity patterns reported here will be altered. 272 (2), 191–194. – All calculations on the metal surfaces and metal clusters were (6) Ma, L.; He, D. Catal. Today 2009, 149 (1 2), 148 156. – performed using gradient-corrected periodic plane wave DFT calcula- (7) Ma, L.; He, D.; Li, Z. Catal. Commun. 2008, 9 (15), 2489 2495. tions as implemented in the Vienna Ab Initio Simulation Program (8) Daniel, O. M.; DeLaRiva, A.; Kunkes, E. L.; Datye, A. K.; – (VASP).37 39 The PW91 form of the GGA was used to determine Dumesic, J. A.; Davis, R. J. ChemCatChem 2010, 2 (9), 1107 1114. (9) Gault, F. G.; Eley, D. D.Pines, H.Weisz, P. B., Eds. Mechanisms gradient corrections to the exchange and correlation energies. Wave of skeletal isomerization of hydrocarbons on metals. In Advances in functions were constructed by expanding a series of plane waves within a Catalysis; Academic Press: New York, 1981; Vol. 30;pp1 95. cutoff energy of 400 eV where interactions between the core and valence 40 (10) Koso, S.; Ueda, N.; Shinmi, Y.; Okumura, K.; Kizuka, T.; electrons were modeled using Vanderbilt ultrasoft pseudopotentials. Tomishige, K. J. Catal. 2009, 267 (1), 89–92. 41 For periodic calculations, a 4 4 1 Monkhorst Pack k-point mesh (11) Kip, B. J.; Hermans, E. G. F.; Van Wolput, J. H. M. C.; Hermans, fi was used to sample the rst Brillouin zone, for the calculations on the N. M. A.; Van Grondelle, J.; Prins, R. Appl. Catal. 1987, 35 (1), 109–139. metal cluster, only the Γ-point was used. The electronic energies ’ (12) Van T Blik, H. F. J.; Niemantsverdriet, J. W. Appl. Catal. 1984, converged to within 1 10 4 eV and forces on each atom were 10 (2), 155–162. optimized to within 0.05 eV Å 1. The 111 surfaces were modeled using a (13) Reyes, P.; Concha, I.; Pecchi, G.; Fierro, J. L. G. J. Mol. Catal. A: 3 3 unit cell with four metal layers and 16 Å vacuum region between Chem. 1998, 129 (2 3), 269–278. each slab where the top two layers were allowed to relax and the bottom two (14) Simonetti, D. A.; Kunkes, E. L.; Dumesic, J. A. J. Catal. 2007, held fixed to their lattice constant for Rh (3.8034 Å). The calculations on 247 (2), 298–306. RhRe clusters were carried out in a 30.4352 Å cubic unit cell to ensure a (15) Augustine, S. M.; Sachtler, W. M. H. J. Catal. 1989, 116 (1), 201 – sufficient vacuum region to separate clusters in neighboring cells; all cluster 184 194. atoms were allowed to relax in energy calculations and transition state (16) Kunkes, E. L.; Simonetti, D. A.; Dumesic, J. A.; Pyrz, W. D.; 42 Murillo, L. E.; Chen, J. G.; Buttrey, D. J. J. Catal. 2008, 260 (1), searches. Transition state searches were performed using the dimer – method with the initial guesses for the transition state structure and the 164 177. (17) Kunimori, K.; Uchijima, T.; Yamada, M.; Matsumoto, H.; reaction trajectory obtained through the nudged elastic band method.43 Hattori, T.; Murakami, Y. Appl. Catal. 1982, 4 (1), 67–81. (18) Gatica, J. M.; Baker, R. T.; Fornasiero, P.; Bernal, S.; Blanco, G.; ’ ASSOCIATED CONTENT Ka spar, J. J. Phys. Chem. B 2000, 104 (19), 4667–4672. (19) Halttunen, M. E.; Niemel€a, M. K.; Krause, A. O. I.; Vaara, T.; bS Supporting Information. Table of the effect of metal- Vuori, A. I. Appl. Catal., A 2001, 205 (1 2), 37–49. loading ratio on catalytic activity and selectivity, table of the effect (20) Sharma, S. B.; Meyers, B. L.; Chen, D. T.; Miller, J.; Dumesic, of pretreatment conditions on hydrogenolysis activity and Re J. A. Appl. Catal., A 1993, 102 (2), 253–265. leaching, table of hydrogenolysis rates and DFT-calculated (21) Niwa, M.; Katada, N.; Sawa, M.; Murakami, Y. J. Phys. Chem. – carbenium and oxocarbenium ion energies, table of hydrogeno- 1995, 99 (21), 8812 8816. (22) Cardona-Martinez, N.; Dumesic, J. A., Thermochemical Char- lysis rates of 1 over 4 wt % Rh ReOx/C (1:0.5) in a continuous fl acterization. In Handbook of Heterogeneous Catalysis; Wiley-VCH Verlag ow reaction system with varying reactant concentrations and fi GmbH & Co. KGaA: Weinheim, 2008; pp 1122 1135. PH2, gure of DFT-calculated metal-catalyzed ring-opening of 3, fi (23) Janik, M. J.; Macht, J.; Iglesia, E.; Neurock, M. J. Phys. Chem. C gure of DFT-optimized structure for water-stabilized tetrahy- 2009, 113 (5), 1872–1885. drofurfuryl carbenium ion transition state. This material is (24) Macht, J.; Janik, M. J.; Neurock, M.; Iglesia, E. J. Am. Chem. Soc. available free of charge via the Internet at http://pubs.acs.org. 2008, 130 (31), 10369–10379. (25) Janik, M. J.; Davis, R. J.; Neurock, M. Catal. Today 2005, 105 ’ AUTHOR INFORMATION (1), 134–143. (26) Gennari, U.; Kramer, R.; Gruber, H. L. Appl. Catal. 1984, 11 Corresponding Author (2), 341–351. [email protected] (27) Gennari, U.; Kramer, R.; Gruber, H. L. Appl. Catal. 1988, 44, 239–250. (28) Boudart, M.; Djega-Mariadassou, G. Kinetics of Heterogeneous ’ ACKNOWLEDGMENT Catalytic Reactions; Princeton University Press: Princeton, NJ, 1984; pp 114 116. This material is based upon work supported by the National (29) Br€andle, M.; Sauer, J. J. Am. Chem. Soc. 1998, 120 (7), Science Foundation under Award No. EEC-0813570 and the 1556–1570. National Science Foundation Partnership in International Re- (30) King, D. L.; Zhang, L.; Xia, G.; Karim, A. M.; Heldebrant, D. J.; search and Education (PIRE) under NSF grant OISE 0730277. Wang, X.; Peterson, T.; Wang, Y. Appl. Catal., B 2010, 99 (1 2), We also kindly acknowledge the computational time at the 206–213. National Center for Computational Sciences at Oak Ridge (31) Shinmi, Y.; Koso, S.; Kubota, T.; Nakagawa, Y.; Tomishige, K. National Laboratory which was used to carry out the simulations Appl. Catal., B 2010, 94 (3 4), 318–326. on the 201 atom RhRe particles. (32) Ma, L.; He, D. H. Top. Catal. 2009, 52 (6 7), 834–844. (33) Spiewak, B. E.; Shen, J.; Dumesic, J. A. J. Phys. Chem. 1995, 99 – ’ (49), 17640 17644. REFERENCES (34) Delley, B. J. Chem. Phys. 1990, 92 (1), 508–517. (1) Chheda, J. N.; Huber, G. W.; Dumesic, J. A. Angew. Chem., Int. (35) MaterialStudio, V. 4.0; Accelyrys, Inc.: San Diego, CA, 2010. Ed. 2007, 46 (38), 7164–7183. (36) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; (2) Schlaf, M. Dalton Trans. 2006, 39, 4645–4653. Pederson, M. R.; Singh, D. J.; Fiolhais, C. Phys. Rev. B 1992, 46 (11), (3) Koso, S.; Furikado, I.; Shimao, A.; Miyazawa, T.; Kunimori, K.; 6671. Tomishige, K. Chem. Commun. 2009, 15, 2035–2037. (37) Kresse, G.; Hafner, J. Phys. Rev. B 1994, 49 (20), 14251.
12688 dx.doi.org/10.1021/ja2038358 |J. Am. Chem. Soc. 2011, 133, 12675–12689 Journal of the American Chemical Society ARTICLE
(38) Kresse, G.; Furthm€uller, J. Phys. Rev. B 1996, 54 (16), 11169. (39) Kresse, G.; Furthm€uller, J. Comput. Mater. Sci. 1996, 6 (1), 15–50. (40) Vanderbilt, D. Phys. Rev. B 1990, 41 (11), 7892. (41) Monkhorst, H. J.; Pack, J. D. Phys. Rev. B 1976, 13 (12), 5188. (42) Graeme, H.; Hannes, J. J. Chem. Phys. 1999, 111 (15), 7010–7022. (43) Jonsson, H.; Mills, G.; Jacobsen, K. W., Nudged Elastic Band Method for Finding Minimum Energy Paths of Transitions; World Scien- tific: Singapore, 1998.
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